1. Field of the Invention
The present invention relates to an optical shifter for use to present a high-resolution image on a projection plane by optically displacing the locations of pixels on the projection plane by a wobbling technique, and also relates to an optical display system including such an optical shifter.
2. Description of the Related Art
A technique of increasing the resolution of an image to be presented by a display panel on a projection plane by optically displacing the locations of pixels on the projection plane (such a technique will be referred to herein as an “image wobbling (or shifting) technique”) is disclosed in Asia Display '95 Digest, pp. 79-82. According to this technique, the apparent locations of pixels being displayed on a projection plane are periodically displaced time-sequentially, thereby increasing the apparent number of pixels. As used herein, the “image” broadly refers to any two-dimensional arrangement of information. Thus, the “images” to be presented on the projection plane include not just images in a narrow sense but also various other types of information such as texts and characters.
A technique of sequentially superimposing three image components, represented by three groups of pixels in the three primary colors of red, green and blue in a display panel, on a projection plane is disclosed in Japanese Laid-Open Publication No. 8-194207, for example. If these three groups of pixels are not wobbled periodically, then each of those pixels will just display red, green or blue. However, this technique realizes a full-color display time-sequentially at each pixel location, thus increasing the resolution of the resultant image displayed.
FIG. 9 shows a conventional optical shifter for use to achieve such image wobbling. As shown in FIG. 9, the optical shifter includes a liquid crystal cell 91 and a birefringent element 92. The liquid crystal cell 91 is disposed at such a position as receiving the incoming light (which position will be referred to herein as “on the light incoming side”), while the birefringent element 92 is disposed at such a position as sending the light out (which position will be referred to herein as “on the light outgoing side). The liquid crystal cell 91 includes a first surface on which the incoming light is incident (which surface will be referred to herein as a “light incoming surface”) and a second surface through which the light leaves the liquid crystal cell (which surface will be referred to herein as a “light outgoing surface”). The liquid crystal cell 91 can change the polarization states of the incoming light according to the voltage applied thereto. More specifically, the liquid crystal cell 91 switches from the state of transmitting the incoming light as it is without rotating the polarization axis thereof at all into the state of rotating the polarization direction thereof by approximately 90 degrees, or vice versa. As used herein, the “polarization direction” means a direction that is perpendicular to the direction in which the light is propagated and that is parallel to the vibration plane of the electric vector.
This switching operation of the liquid crystal cell 91 is controlled by the level of the voltage to be applied to the liquid crystal layer of the liquid crystal cell 91. Suppose a polarized light ray having a polarization direction that comes out of the paper is incident onto the first surface of the liquid crystal cell 91 as shown in FIG. 9. In the example shown in FIG. 9, if the voltage being applied to the liquid crystal cell 91 is in OFF state (i.e., when zero voltage is being applied to the liquid crystal layer), then a polarized light ray, of which the polarization axis has been rotated approximately 90 degrees, goes out through the second surface of the liquid crystal cell 91. In that case, the light ray that has gone out through the second surface has a polarization axis that is parallel to the paper as shown in FIG. 9. On the other hand, if the voltage being applied to the liquid crystal cell 91 is in ON state (e.g., when a voltage of 5 V is being applied to the liquid crystal layer), then a polarized light ray, of which the polarization axis has not been rotated at all, goes out through the second surface of the liquid crystal cell 91. In that case, the light ray that has gone out through the second surface has a polarization axis that is still perpendicular to the paper as shown in FIG. 9. The liquid crystal layer of the liquid crystal cell 91 may operate in any of various known liquid crystal display modes including the twisted nematic (TN) mode, optically compensated birefringence (OCB) mode and ferroelectric liquid crystal (FLC) mode.
The birefringent element 92 selectively shifts the optical axis of an incoming linearly polarized light ray depending on the polarization direction thereof. For that purpose, the birefringent element 92 is made of a uniaxial crystalline material exhibiting birefringence and having a thickness t.
In the example shown in FIG. 9, while the voltage being applied to the liquid crystal cell 91 is in OFF state, the light ray that has passed through the birefringent element 92 is shifted as an extraordinary ray. On the other hand, while the voltage being applied to the liquid crystal cell 91 is in ON state, the light ray that has passed through the birefringent element 92 is not shifted at all because that light ray is an ordinary ray. The magnitude of this shifting can be regulated by the thickness t of the birefringent element 92.
Examples of preferred materials for the birefringent element 92 include quartz, lithium niobate, calcite, mica, rutile (TiO2) and nitratine (NaNO3). If the total weight of the display system should be reduced as in a head mounted display (HMD), lithium niobate or rutile having relatively large refractive index anisotropy Δn is preferably used. When the birefringent element 92 is made of such a high Δn material, the minimum required image shift is realized by the birefringent element 92 with a reduced thickness. Thus, such a material can be used effectively to reduce the overall size or weight of the display system.
One of the problems that optical display systems of this type (i.e., attempting to increase the resolution by utilizing the wobbling technique) commonly have is that the resultant image quality heavily depends on the response characteristic of the liquid crystal cell 91 included in the optical shifter. The response characteristic of the liquid crystal cell 91 in turn exhibits temperature dependence. For these reasons, to achieve good response characteristics, the temperature of the liquid crystal cell 91 needs to be maintained within an appropriate range.
A conventional technique of controlling the temperature of the liquid crystal cell is described in Japanese Laid-Open Publication No. 11-326877, for example. According to that technique, the liquid crystal cell is directly surrounded with, and heated by, a heater. The conventional heating technique will be described with reference to FIG. 10.
As shown in FIG. 10, the conventional optical display system includes: a display panel 93; a liquid crystal cell 91 that selectively changes the polarization states of the outgoing light of the display panel 93 according to the voltage applied to the cell 91; and a birefringent element 92 for selectively shifting the optical path of the light that has gone out of the liquid crystal cell 91 depending on the polarization state thereof. The display operation of the display panel 93 is controlled by a display controller 95.
In the example shown in FIG. 10, a heater 94 is provided around the side surfaces of the liquid crystal cell 91 to maintain the temperature of the liquid crystal cell 91 within a predetermined range and thereby keep the response characteristic of the liquid crystal cell 91 good enough. A temperature sensor (not shown) is provided near the heater 94. In accordance with the output of the temperature sensor, a heater driver/controller 96 controls the ON/OFF states of the heater 94 (i.e., selectively energizes the heater 94).
However, if the liquid crystal cell 91 is heated inward (i.e., from around the liquid crystal cell 91) by the heater 94 that surrounds the liquid crystal cell 91, then the two-dimensional temperature distribution of the liquid crystal cell 91 might become non-uniform depending on the size of the light incoming and outgoing surfaces (i.e., the two surfaces of the liquid crystal cell 91 through which the incoming light passes). More specifically, when the liquid crystal cell 91 is heated by the surrounding heater 94, the temperature rises more easily at the peripheral portions of the liquid crystal cell 91 than at the center portion thereof. As a result, the response speed of liquid crystal molecules will have a non-uniform two-dimensional distribution in the liquid crystal cell 91. Then, the resultant image quality will deteriorate significantly.
The conventional optical display system shown in FIG. 10 also has another problem. Specifically, in this display system, the liquid crystal cell 91 itself is approximately as thin as two glass plates. Thus, it is actually difficult to equip that thin liquid crystal cell 91 with the heater 94.
Japanese Laid-Open Publication No. 11-326877 identified above also discloses a technique of heating the overall liquid crystal cell uniformly by providing a heater pattern (made of a transparent conductive film) on the glass substrate of the cell.
In this method, the liquid crystal cell can have a substantially uniform two-dimensional temperature distribution, thus causing almost no variation in response speed. However, since the transparent conductive film is inserted into the optical path, the transmittance of the light should decrease.
FIG. 11 shows an alternative technique of using one of two transparent electrodes, which should be provided for the liquid crystal cell to drive the liquid crystal layer, as a heater by creating a voltage VH between two terminals of the transparent electrode such that a current flows through a plane of that transparent electrode. In this method, the transparent conductive film that is indispensable to operate the liquid crystal cell can be used effectively as a heater. That is to say, since no additional transparent conductive film pattern is provided, the transmittance of the light does not decrease. In this configuration, however, not only the voltage VH needs to be created between the two terminals of the transparent electrode to generate the heat, but also another voltage VLC needs to be created between the pair of transparent electrodes to drive the liquid crystal layer. Thus, a gradient will be produced in the voltage that is actually applied to the liquid crystal layer. As a result, the response characteristic of liquid crystal molecules might have a non-uniform two-dimensional distribution.
It should be noted that if multiple pairs of liquid crystal cells and birefringent elements are used in combination, then the image can be selectively shifted to one of three or more locations on a projection plane. In that case, however, it is necessary to equalize the response speeds of the respective liquid crystal cells with each other as opposed to the situation where just one liquid crystal cell is used. This is because if the respective liquid crystal cells have mutually different response speeds, then the image shifting timing will become inconsistent and the resultant image quality will deteriorate. For that reason, the temperatures of all liquid crystal cells must be kept uniform in that case. However, Japanese Laid-Open Publication No. 11-326877 identified above provides no solutions for this problem.